This invention relates to a method of manufacturing a double-ended lamp; it is particularly suitable for making high pressure metal vapor discharge lamps of the type wherein the lamp body comprises an enlarged bulbous midportion with oppositely extending tubular necks. The bulbous midportion defines an arc chamber which contains an inert starting gas and a fill of vaporizable metal or metals such as mercury or a mixture of mercury and selected metal halides. Electrodes are hermetically sealed within the necks and project into the arc chamber. When an arc is created across the interelectrode gap and the fill is vaporized within the arc chamber, light is produced in known manner.
The invention relates more particularly to the manufacture of a lamp of the type in which the fill and the starting gas are introduced into the arc chamber through one of the necks. This is in contrast to a lamp in which the electrodes are first sealed within the necks and in which the fill and the starting gas are then introduced into the arc chamber through a lateral exhaust tube which is subsequently tipped off. The vestiges of the tip-off form a discontinuity which is more objectionable the smaller the size of the lamp.
In all discharge lamps it is necessary to have a clean arc chamber and to avoid contamination of the fill. Certain types of fills (e.g., those containing metal halides) are very hygroscopic and react when exposed to even minute amounts of water vapor. Metal halides usually are supplied in the form of pellets having a high degree of purity. To preserve this purity and insure the production of an acceptable lamp, it is necessary to protect the halides, the electrodes and the lamp body from water vapor and other contaminants during assembly of the components and until such time as the arc chamber has been safely sealed. In miniature discharge lamps, the deleterious effects of contamination are magnified and the need for prorection is all the greater.
In high pressure metal vapor lamps, the arc voltage drop varies proportionally to the length of the interelectrode gap. The heating of the ends of the arc chamber is strongly influenced by the extent to which the electrodes are inserted and project into the chamber. Such heating determines vaporization of the fill, particularly the metal halides which tend to condense in the cooler ends. Thus both the length and the location of the interelectrode gap are important and the need for precision in its determination increases as the size of the lamp is reduced.